What causes scum on tea?

The main culprit behind the formation of scum or film on the surface of tea is the presence of tannins, which are polyphenols found naturally in tea leaves. These tannins, when interacting with other compounds present in the tea leaves, can create a whitish foam or cloudy appearance on the surface.

Tannins are more abundant in certain types of tea, especially black tea. The oxidation process that occurs during the production of black tea leads to increased tannin content, hence the propensity for forming scum. Factors like the tea quality, processing methods, water quality, and brewing conditions can also influence the formation of scum.

When tea leaves or tea bags are steeped in hot water, the tannins and other compounds are released and dispersed into the liquid. As the tea cools down, the tannins can interact with other compounds such as proteins, minerals, and caffeine to create a complex mixture. This interaction leads to the formation of insoluble compounds that float to the surface, creating the visible scum or film.

To reduce the appearance of scum on tea, consider using higher-quality tea leaves or tea bags and brewing the tea with fresh, filtered water. Additionally, avoiding over-steeping the tea can help prevent the release of excessive tannins. Some people prefer to rinse the tea leaves with hot water before brewing, as it helps remove surface dust and reduce the amount of scum.